1. Field of the Invention
The present invention relates to a process and an apparatus for chemical vapor deposition.
2. Description of the Related Art
The chemical vapor deposition (CVD) or vapor phase epitaxy (VPE) process is widely used in the semiconductor industry. The basic principle of CVD is that a reactant gas or gases are introduced into a reaction furnace or chamber of a CVD apparatus through an inlet port and an energy source such as heat, light, plasma, etc., is utilized to cause a chemical reaction and thereby deposit a semiconductor, a metal, an insulating substance, etc., on a substrate, and a used gas produced by the chemical reaction is immediately exhausted out of the CVD apparatus through one or more exhaust ports of the reaction furnace.
FIG. 1 schematically illustrates various types of CVD apparatuses currently in use, including (a) a vertical type, (b) a pancake type, (c) a barrel type, and (d) a horizontal type, in which "G" denotes an introduced reactant gas and "E" an exhaust gas.
The provision of a uniform flow of reactant gas(es) in the reaction furnace of a CVD apparatus is extremely important, to obtain a uniformly deposited layer or film, and the uniformity of the reactant gas flow in a reaction furnace depends significantly on the uniformity of the exhausting of a used gas through a plurality of exhaust ports.
A mass flow controller (MFC) is generally used to ensure a precise control of the flow of fluid. FIG. 2 shows an essential structure of an MFC.
As shown in the figure, a narrow bypass 102 for detecting the flow quantity branches from and rejoins a main flow path 101. A fluid flowing through the bypass 102 is heated by a heater 103, and the temperature distribution in the fluid is measured by temperature sensors 104a and 104b disposed upstream and downstream of the heater 103, respectively. When the flow quantity is zero, the upstream and the downstream temperature distributions are equal, and as the flow quantity is increased, the upstream temperature distribution is lowered and the downstream temperature distribution is raised. This relationship enables the flow quantity to be precisely determined. The thus-measured value in the form of an electric signal is fed to a control circuit 105, where the measured value is compared with a preset flow quantity value, and a control signal corresponding to the difference between these two values is transmitted to a drive motor 107 of an electric valve 106 inserted in a downstream portion of the main flow path 101, to adjust the valve 106 so that the flow quantity is kept at the preset value. An element 108 for ensuring a laminar flow provides a resistance to the fluid flow, to cause a pressure difference between both sides of the element 108. This pressure differential causes the fluid flow to branch off the main path 101 into the bypass 102.
An MFC having the above-mentioned structure cannot be practically used when adjusting the exhaust flow quantity of a CVD apparatus, for the following reason.
The exhaust flow from a CVD apparatus carries various byproducts resulting from the formation reaction of a deposit layer, and these byproducts adhere to the inside walls of a reaction furnace and an exhaust pipe, and thereafter, fall from the walls to form a flaky dust, which is carried by the exhaust flow. The bypass 102 for detecting the flow quantity, however, is very narrow, i.e., usually has a diameter of from about 0.2 to 0.5 mm, to ensure a high measuring sensitivity, and the flaky dust entrained in the exhaust flow and entering the narrow bypass 102 causes a blockage or choking of the bypass 102, to thereby make the measuring of flow quantity impossible, and consequently, the MFC cannot control the flow quantity. Accordingly, MFCs are not suitable for adjusting the exhaust flow of a CVD apparatus.
Therefore, various measures have been taken with regard to the exhaust ports of a CVD apparatus to provide a uniform gas flow in the reaction furnace.
FIGS. 3(a) and 3(b) show conventional vertical type CVD apparatuses in a plan view (a) and a perspective view (b), respectively. The apparatus shown in FIG. 3(a) is provided with exhaust pipes 741 and 742 having a uniform shape in the portion between a not-shown exhauster and two exhaust ports 731 and 732 of a furnace 710, and therefore, having a uniform conductance. In this arrangement, the quantities of two exhaust gas flows from the two exhaust ports 731 and 732 are equal, and thus the gas flow in the furnace is symmetrical. This conventional arrangement, however, has a drawback in that the arrangement of such an apparatus is very limited and has a very poor flexibility. Namely, first the positions of a not-shown exhauster and a reaction furnace 710 must be fixed. Moreover, when the number of exhaust ports is increased to three or four, to improve the uniformity of gas flow in the furnace, the piping around the furnace becomes complicated. The apparatus shown in FIG. 3(b) has a single exhaust port 731 and is provided with a plate 701 through which a plurality of openings 702 having different diameters are formed to adjust the conductance, i.e., the closer to the exhaust port, the smaller the opening diameter, to thereby provide a uniform gas flow in the furnace, and vice versa. To determined the size and position of such openings, a number of experiments must be carried out by actually operating the furnace with a gas flowing therethrough, which require a great deal of time and labor.